Prior-art locking mechanisms of this type are usually operated by means of a control screw, by whose means the mobility and thereby the foldability of a piece of seating furniture can be locked and gradually released. It is a drawback of this prior-art type of locking mechanism that from the lowered rest position the piece of seating furniture cannot be brought back to the normal sitting position without first releasing the locking mechanism, i.e. screwing on the control handle. When the piece of seating furniture is in the rest position, rising from this is quite difficult, because its seat part has then been pushed forward and is in a markedly raised position, at the same time as the back part has been lowered strongly rearwards.